Parts of the UK could see two weeks of average rainfall in an hour as the first major summer downpours hit.
Severe weather warnings have been issued covering much of the country amid fears of flooding.
Some places could see between 0.8in (20mm) and 1.2in (30mm) of rainfall in an hour - almost half of the UK monthly average for June of 2.9in (73.4mm).
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Regions where yellow "be aware" warnings for rain are in place are Strathclyde; Central, Tayside and Fife; south-west Scotland; Lothian Borders; Northern Ireland; the north of England and Wales.
Met Office spokesman Dan Williams said the downpours will be "pretty heavy".
He said: "We are looking at the potential for some localised flooding because they are potentially heavy enough that we could see so much rain fall in a short space of time that it can't drain away fast enough."
The predicted deluge is because of warm, humid air moving in from Europe which creates a risk of downpours when it mixes with cooler air over the UK.
It has been a soggy 2014 so far, particularly in Reading.
Friends of the Earth climate campaigner Guy Shrubsole said: "After the wettest winter ever recorded it's clear something is seriously weird with our weather.
"With the Met Office confirming this week climate change is set to make heavy summer downpours the norm in future, the Government must take this threat far more seriously.
"It should start by putting carbon-cutting at the heart of policy-making, and building enough flood defences to keep pace with our changing climate - rather than having to splash out to repair the damage later on."